Recently, the display performances of liquid crystal display devices have been improved to the point that more and more manufacturers use them in TV receivers, for example. The viewing angle characteristic of liquid crystal display devices has been improved to a certain degree but is not satisfactorily in some respects. Among other things, there is still a high demand for improvement of the viewing angle characteristic of a liquid crystal display device that uses a vertical alignment liquid crystal layer. A liquid crystal display device with a vertical alignment liquid crystal layer is sometimes called a “VA mode liquid crystal display device”.
A VA mode liquid crystal display device which is currently used for a TV set with a big screen, for example, adopts an alignment division structure in which multiple liquid crystal domains are formed in a single pixel to improve the viewing angle characteristic. An MVA (multi-domain vertical alignment) mode is often adopted as a method of forming such an alignment division structure. The MVA mode is disclosed in Patent Document No. 1, for example.
Specifically, according to the MVA mode, an alignment control structure is provided on each of the two substrates, which face each other with a vertical alignment liquid crystal layer interposed between them, so as to contact with the liquid crystal layer, thereby forming multiple liquid crystal domains with mutually different alignment directions (i.e., tilt directions), the number of which is typically four, in each pixel. As the alignment control structure, a slit (as an opening) or a rib (as a projection structure) may be provided for an electrode, thereby creating an alignment controlling force from both sides of the liquid crystal layer.
If such a slit or rib is adopted, however, the alignment controlling force will be applied onto liquid crystal molecules non-uniformly within a pixel because the slit or rib has a linear structure unlike the situation where the pretilt directions are defined by an alignment film in a conventional TN (twisted nematic) mode LCD. As a result, the response speed may have a distribution unintentionally. In addition, since the transmittance of light will decrease in the areas with the slits or ribs, the brightness of the screen will decrease, too.
To avoid such a problem, the alignment division structure is suitably formed by defining the pretilt directions with an alignment film for a VA mode liquid crystal display device, too. A VA mode liquid crystal display device, of which the alignment control structure has been formed in this manner, is proposed in Patent Document No. 2.
In the liquid crystal display device disclosed in Patent Document No. 2, a quadruple alignment division structure is formed by defining the pretilt direction with an alignment film. That is to say, when a voltage is applied to the liquid crystal layer, four liquid crystal domains are formed in a single pixel. Such a quadruple alignment division structure will be sometimes simply referred to herein as a “4D structure”.
Also, in the liquid crystal display device disclosed in Patent Document No. 2, the pretilt direction defined by one of the two alignment films that face each other with the liquid crystal layer interposed between them and the pretilt direction defined by the other alignment film are different from each other by substantially 90 degrees. That is why when a voltage is applied, the liquid crystal molecules will have a twisted alignment. Such a VA mode in which the liquid crystal molecules have a twisted alignment by using two alignment films which are arranged so that their pretilt directions (alignment treatment directions) are perpendicular to each other is sometimes called either a VAIN (vertical alignment twisted nematic) mode or an RTN (reverse twisted nematic) mode. As already described, the 4D structure is formed in the liquid crystal display device of Patent Document No. 2, and therefore, the display mode of the liquid crystal display device of Patent Document No. 2 is sometimes called a “4D-RTN” mode.
As for a specific method for getting the pretilt directions of the liquid crystal molecules defined by alignment films, a method that uses a photo-alignment treatment is thought to be a promising one as pointed out in Patent Document No. 2. The photo-alignment treatment contributes to increasing the yield because that treatment is a non-contact method and generates no static electricity due to friction unlike the rubbing treatment. An alignment film subjected to such a photo-alignment treatment is sometimes called a “photo-alignment film”.
Also, in order to improve the responsivity of the MVA mode, so-called “PSA (polymer sustained alignment) technology” has been developed recently. The PSA technology is disclosed in Patent Documents Nos. 3 and 4. According to the PSA technology, to give a pretilt to liquid crystal molecules, a polymer layer, which is called an “alignment sustaining layer”, is used. The alignment sustaining layer is formed by polymerizing a photo-polymerizable monomer, which has been added in advance to the liquid crystal material, with a voltage applied to the liquid crystal layer after a liquid crystal cell is completed. By adjusting the distribution and intensity of an electric field to be applied to polymerize the monomer, the pretilt azimuth (i.e., azimuth angle within a substrate plane) and pretilt angle (i.e., an elevation angle with respect to the substrate plane) of the liquid crystal molecules can be controlled.
Meanwhile, Patent Document No. 4 discloses a configuration in which a pixel electrode with a fine-line striped pattern (which is sometimes called a “fishbone type pixel electrode”) is used in combination with the PSA technology. According to such a configuration, when a voltage is applied to the liquid crystal layer, the liquid crystal molecules will be aligned parallel to the longitudinal direction of the striped pattern, which is in sharp contrast to the conventional MVA mode disclosed in Patent Document No. 1 in which the liquid crystal molecules are aligned perpendicularly to the linear alignment control structure such as slits or ribs. The lines and spaces of the fine-line striped pattern may have a narrower width than the conventional MVA mode alignment control structure. That is why the fishbone type pixel electrode is applicable more easily to small pixels than the conventional MVA mode alignment control structure is.
According to these modified VA mode technologies (including the 4D-RTN mode, the PSA technology, and the fishbone type pixel electrode), an excellent viewing angle characteristic is realized. Recently, however, since there is a growing demand for further improvement of the viewing angle characteristic of VA mode liquid crystal display devices, a so-called “pixel division driving technique” has been incorporated into actual products one after another (see Patent Documents Nos. 5 and 6, for example).
According to the pixel division driving technique, the phenomenon that the γ (gamma) characteristic when the screen is viewed straight on is different from the γ (gamma) characteristic when the screen is viewed obliquely, i.e., the viewing angle dependence of the γ characteristic, can be significantly reduced. In this case, the γ characteristic is the grayscale dependence of the display luminance.
Also, according to the pixel division driving technique, a single pixel is comprised of a plurality of subpixels which can apply mutually different voltages to the liquid crystal layer (i.e., which can exhibit mutually different luminances), and a predetermined luminance corresponding to the display signal voltage to be input to a pixel is realized by the entire pixel. That is to say, the pixel division driving technique is a technique for reducing the viewing angle dependence of a pixel's γ characteristic by synthesizing together mutually different γ characteristics of a plurality of subpixels.